Method and aparatus for monitoring a junction between electrical devices

ABSTRACT

A method and a test fixture for evaluating a junction between an electrical lead trace and a busbar are described, and include an electric power supply disposed to supply electric power to the electrical lead trace and an electric monitoring device disposed to monitor electrical potential across the junction. A mechanical stress-inducing device is disposed to apply mechanical stress proximal to the junction. The electric monitoring device monitors the electrical potential across the junction of the electrical lead trace coincident with the mechanical stress-inducing device applying mechanical stress proximal to the junction when the electric power supply is supplying electric power to the electrical lead trace. Electrical integrity of the junction is evaluated based upon the monitored electrical potential across the junction.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication No. 62/331,659 filed on May 4, 2016, the disclosure of whichis hereby incorporated by reference.

TECHNICAL FIELD

This disclosure relates to junctions between electrical devices, andmonitoring thereof.

BACKGROUND

A battery pack typically includes multiple rechargeable battery cellsthat are connected in series or parallel to store and supply electricpower to a distribution system. Terminals of adjacent battery cells arejoined at busbars, and electrical lead traces may electrically connectto the busbars for monitoring purposes. Joining of electrical leadtraces to busbars may be accomplished employing a mechanical fastener,e.g., a rivet. There are opportunities for improvement of methods andtest fixtures to evaluate electrical integrity of such a mechanicalfastener at assembly. Known methods for evaluating electrical integrityinclude static impedance tests and visual inspections.

SUMMARY

A method and a test fixture for evaluating a junction between anelectrical lead trace and a busbar are described, and include anelectric power supply disposed to supply electric power to theelectrical lead trace and an electric monitoring device disposed tomonitor electrical potential across the junction. A mechanicalstress-inducing device is disposed to apply mechanical stress proximalto the junction. The electric monitoring device monitors the electricalpotential across the junction of the electrical lead trace coincidentwith the mechanical stress-inducing device applying mechanical stressproximal to the junction when the electric power supply is supplyingelectric power to the electrical lead trace. Electrical integrity of thejunction is evaluated based upon the monitored electrical potentialacross the junction.

The above features and advantages, and other features and advantages, ofthe present teachings are readily apparent from the following detaileddescription of some of the best modes and other embodiments for carryingout the present teachings, as defined in the appended claims, when takenin connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

One or more embodiments will now be described, by way of example, withreference to the accompanying drawings, in which:

FIG. 1 is a schematic top plan view of a battery cell interconnect boardincluding a plurality of electrically conductive busbars disposed in aframe, in accordance with the disclosure;

FIG. 2 schematically shows a test fixture associated with evaluating oneof the electrical lead traces described with reference to FIG. 1,including an electric power supply, an electric monitoring device, amechanical stress-inducing device and a controller in accordance withthe disclosure;

FIGS. 3-1 and 3-2 graphically show results associated with employing thetest fixture described with reference to FIG. 2 to evaluate one of theelectrical lead traces described with reference to FIG. 1, in accordancewith the disclosure; and

FIG. 4 schematically shows a plan view of an electrical circuit for anintegrated test fixture and selected elements of the battery cellinterconnect board that is described with reference to FIG. 1, inaccordance with the disclosure.

DETAILED DESCRIPTION

Referring now to the drawings, which are provided for the purpose ofillustrating certain exemplary embodiments only and not for the purposeof limiting the same, FIG. 1 schematically illustrates a battery cellinterconnect board 10 including a plurality of electrically conductivebusbars 30 that are disposed in a non-conductive frame 12, and anaccompanying monitoring circuit 40 that includes an electrical connector50. The battery cell interconnect board 10 is advantageously disposed ona second frame portion (not shown) that houses a plurality ofrechargeable battery cells (not shown). The battery cell interconnectboard 10 and the second frame portion together form a battery pack. Thebattery cells each have a positive terminal and a negative terminal, andsubsets of the positive or negative terminals are welded to one of thebusbars 30. The battery cells may be connected in series or parallelthrough the battery cell interconnect board 10 to store and supplyelectric power to an electric power distribution system. The batterypack may be disposed on a vehicle in one embodiment to supply electricpower to propulsion systems and other systems, depending upon thespecific application. Those having ordinary skill in the art willrecognize that terms such as “horizontal”, “vertical”, “above,” “below,”“upward,” “downward,” “top,” “bottom,” etc., are used descriptively forthe figures, and do not represent limitations on the scope of thedisclosure, as defined by the appended claims. Furthermore, thedisclosure, as illustrated and described herein, may be practiced in theabsence of any element which is not specifically disclosed herein.

Each busbar 30 is fabricated from conductive material, e.g., copper, andpreferably includes a web portion 31 and side portions 32, with a firstaperture 33 and a second aperture 34 formed in the web portion 31. Thefirst aperture 33 is preferably centrally disposed on the web portion 31along its longitudinal axis, and the second aperture 34 is preferablydisposed near one of the ends of the web portion 31. Subsets of thepositive or negative terminals of the battery cell are welded to sideportions 32 of the busbars 30.

The battery cell interconnect board 10 includes the electricallyconductive busbars 30 disposed in the non-conductive frame 12. The frame12 may be a rectangularly-shaped rigid device that is fabricated fromnon-conductive thermoplastic material, e.g., polycarbonate, usinginjection molding or another suitable process. The frame 12 includes anouter peripheral portion 14, a central bridge 16, a plurality ofreinforcement ribs 19 and a plurality of side bridges 20. A plurality ofapertures 18 are formed between adjacent reinforcement ribs 19 and sidebridges 20. The apertures 18 accommodate either the positive terminalsor the negative terminals from a subset of the battery cells, whichextend therethrough to permit welding to the side portions 32 of one ofthe busbars 30 during a subsequent assembly process. In one embodiment,the battery cell interconnect board 10 is formed by overmolding theframe 12 onto the plurality of busbars 30, wherein each of the busbars30 is oriented with the second aperture 34 proximal to the centralbridge 16. Each of the busbars 30 preferably includes protrusionportions that secure the busbars 30 into the frame 12 as part of theovermolding process. Alternatively, the frame 12 molded such that eachof the side bridges 20 includes a protrusion portion 22 that extendsupwardly from the surface of the side bridge 20. The busbars 30 areassembled onto the side bridges 20 such that the web portion 31 of eachbusbar 30 is contiguous with the side bridge 20. Each busbar 30 isoriented to have its second aperture 34 proximal to the central bridge16, and the protrusion portion 22 of the side bridge 20 is inserted intothe first aperture 33. Heat is then applied to plastically deform theprotrusion portion 22 and fixedly secure the busbar 30 to the sidebridge 20.

The monitoring circuit 40 includes a plurality of electrical lead traces44 that are fabricated onto a non-conductive flexible web material 45,wherein the electrical lead traces 44 electrically connect between thebusbars 30 and terminal pins 52 of the connector 50 for monitoring andsignal communication. One of the electrical lead traces 44 electricallyconnects between one of the busbars 30 and one of the terminal pins 52of the connector 50, and preferably includes an in-series fuse 46. Asingle electrical lead trace 44, fuse 46 and terminal pin 52 are shownfor ease of illustration. In use, the connector 50 communicateselectrical information gathered from the subsets of the battery cellsvia the busbars 30 for purposes of monitoring, load balancing, faultdetection, etc. The electrical connector 50 includes a plurality ofterminal pins 52 that are arranged in a structured body to effectconnection to another device, such as a monitoring controller.

A portion of each electrical lead trace 44 and a portion of the webmaterial 45 is formed into a tab 42 that preferably overlaps with one ofthe busbars 30 such that the tab 42 is adjacent with the second aperture34 of the web portion 31 of the busbar 30. The tab 42 is fixedly securedto the busbar 30 employing a permanent mechanical fastener 36, such as arivet, which forms a junction 38 between the tab 42 and the busbar 30.Each mechanical fastener 36 forms the junction 38 by applying a normalforce that compresses adjoining surfaces of the busbar 30 and theportion of the electrical lead trace 44 formed into the tab 42. Thenormal force may be applied by deforming a portion of the mechanicalfastener 36 in one embodiment. Each junction 38 has two components,including a mechanical joining of the tab 42 and the busbar 30, and anelectrically-conductive joining of the portion of the electrical leadtrace 44 and the busbar 30. Mechanical fasteners 36 such as rivets areknown.

Assembly processes associated with mechanically coupling a plurality ofthe tabs 42 to corresponding busbars 30 employing a plurality ofmechanical fasteners 36 may be subject to variation. Such variation maybe associated with the magnitude of the applied normal force due to thedeformation of the mechanical fastener 36 during fabrication, whereinthe variation may be non-obvious. Furthermore, one of the junctions 38formed between one of the tabs 42 and one of the busbars 30 may appearto be mechanically sound but have a non-obvious difference thatintroduces variation in the electrical conductivity across the junction38. This variation in the electrical conductivity may be immediatelydiscernible, may be discernible after time and use, or may bediscernible in response to an induced stress.

Each of the electrical lead traces 44 has a characteristic resistancethat is determined based upon the trace length and pattern on theflexible web material 45, the fuse 46, solder and/or other interfaces,the terminal pin 52 and the junction 38 formed between the tab 42 andthe busbar 30 by the mechanical fastener 36. The characteristicresistance may vary due to variation resistance at the junction 38,which may depend upon the magnitude of normal force that is applied bythe mechanical fastener 36 to form the junction 38. By way of anon-limiting example, if a rivet is not formed properly during assembly,the rivet may have low clamping force, which may increase resistance atthe junction 38.

FIG. 2 schematically shows a test fixture 75 associated with evaluatingone of the electrical lead traces 44, and more specifically evaluatingthe electrical conductivity of one of the junctions 38 formed betweenthe busbar 30 and the electrical lead trace 44 by the mechanicalfastener 36. The junction 38 and an accompanying in-line fuse 46 areevaluated as resistive devices. The test fixture 75 is preferablyconfigured to non-destructively evaluate the junction 38 formed betweenthe busbar 30 and the electrical lead trace 44 by the mechanicalfastener 36 on a workpiece. The test fixture 75 includes an electricpower supply 80, an electric monitoring device 85, a mechanicalstress-inducing device 90, and an associated controller 95. Overall, thecontroller 95 employs the test fixture 75 to measure a dynamic change inresistance at the junction 38 while the mechanical fastener 36 issubjected to an external non-destructive mechanical stress.

The electric power supply 80 is preferably a low-power device thatelectrically connects to a workpiece in the form of one of theelectrical lead traces 44 to supply electric power at a preset voltagelevel. A load resistor 82 is placed in series with the electrical leadtrace 44 to limit the current. The load resistor 82 is preferablyselected to facilitate detecting a change in the monitored voltage thatmay occur due to a change in the overall resistance of the electricallead trace 44, wherein the change in the monitored voltage may be causedby a change in the resistance across the junction 38. In onenon-limiting embodiment, the preset voltage level is 0.5 volts DC.

The electric monitoring device 85 may be a voltmeter that isincorporated into a digital data acquisition device that monitorsvoltage, e.g., at a 1 kHz rate. The electric monitoring device 85 ispreferably disposed to monitor a voltage between leads 86 and 87, whichthus provides a voltage drop across the junction 38 and the in-line fuse46. Connection to the lead 86 may be made via a pogo pin, and connectionto the lead 87 may be made via the corresponding terminal pin 52 in theelectrical connector 50.

The mechanical stress-inducing device 90 may be a device that isconfigured to apply mechanical stress proximal to the junction 38. Themagnitude of the mechanical stress is sufficient to induce a discerniblechange in electrical resistance across the junction 38 when the junction38 was not formed in accordance with specification, but limited so as tonot induce new stress in the junction 38. The mechanical stress may bein the form of a burst of pressurized airflow that is applied to thejunction 38, e.g., from a high-pressure source via a nozzle that isaimed towards the junction 38. Alternatively, the mechanical stress maybe in the form of a direct mechanical tapping onto the junction 38,e.g., from a hammer device that is disposed to tap on the junction 38.Alternatively, the mechanical stress may be in the form of an inducedvibration at the junction 38, e.g., from a horn of an ultrasonic weldingdevice that is placed in contact with the junction 38. Alternatively,the mechanical stress may be in the form of another suitable mechanicalstress.

The test fixture 75 preferably includes a controller 95 that executesone or more control routines to evaluate the integrity of thecorresponding junction based upon the monitored electrical conductivitywhen applying the mechanical stress, which may be described as follows.In operation, one of the electrical lead traces 44 is secured andelectrically connected to the electric power supply 80 and the electricmonitoring device 85 of the test fixture 75. The mechanicalstress-inducing device 90 is activated and the electric monitoringdevice 85 monitors voltage across the junction 38. The monitored voltageis evaluated. When there is a change in the monitored voltage associatedwith the induced stress, it may be indicative of an increased resistancein the junction 38 caused when the junction 38 was not formed inaccordance with specification.

By way of a non-limiting example, a resistance level associated with anembodiment of the junction 38 that was formed in accordance with thespecification may be in the order of magnitude of 2 milli-ohms, and aresistance level associated with an embodiment of the junction 38 thatwas not formed in accordance with the specification may be in the orderof magnitude of 50 milli-ohms when mechanical stress is induced. Assuch, the electric monitoring device 85 must be configured to detect anddiscern between such levels. Preferably, there is a threshold voltagelevel, with minor changes in the increased resistance in the junction 38not causing rejection of a workpiece. When there is no change in themonitored voltage associated with the induced stress, it may indicatethat the junction 38 was formed in accordance with the specification.

FIGS. 3-1 and 3-2 graphically show results associated with employing thetest fixture 75 described with reference to FIG. 2 to evaluate one ofthe electrical lead traces 44 described with reference to FIG. 1.Voltage levels indicated by the electric monitoring device 85 are shownon the vertical scale 302, in relation to time, which is shown on thehorizontal scale 304. The embodiment of the mechanical stress-inducingdevice 90 is in the form of a burst of pressurized airflow that isapplied to the junction 38. The voltage spikes 310 shown in FIG. 3-1include a plurality of voltage spikes 312, which are associated withrepeated operation of an embodiment of the mechanical stress-inducingdevice 90 described with reference to FIG. 2, wherein the spikes 312 mayindicate an increased resistance in the junction 38 caused when thejunction 38 was not formed in accordance with specification. The voltagelevels 320 shown in FIG. 3-2 do not include any voltage spikes that areassociated with repeated operation of an embodiment of the mechanicalstress-inducing device 90 described with reference to FIG. 2. This mayindicate no increased resistance in the junction 38, i.e., the junction38 was formed in accordance with specification and the workpiece may bedeemed acceptable.

The terms controller, control module, module, control, control unit,processor and similar terms refer to any one or various combinations ofApplication Specific Integrated Circuit(s) (ASIC), electroniccircuit(s), central processing unit(s), e.g., microprocessor(s) andassociated non-transitory memory component in the form of memory andstorage devices (read only, programmable read only, random access, harddrive, etc.). The non-transitory memory component is capable of storingmachine readable instructions in the form of one or more software orfirmware programs or routines, combinational logic circuit(s),input/output circuit(s) and devices, signal conditioning and buffercircuitry and other components that can be accessed by one or moreprocessors to provide a described functionality. Input/output circuit(s)and devices include analog/digital converters and related devices thatmonitor inputs from sensors, with such inputs monitored at a presetsampling frequency or in response to a triggering event. Software,firmware, programs, instructions, control routines, code, algorithms andsimilar terms mean any controller-executable instruction sets includingcalibrations and look-up tables. Each controller executes controlroutine(s) to provide desired functions, including monitoring inputsfrom sensing devices and other networked controllers and executingcontrol and diagnostic instructions to control operation of actuators.Routines may be executed at regular intervals, for example each 100microseconds during ongoing operation. Alternatively, routines may beexecuted in response to occurrence of a triggering event. Communicationbetween controllers, and communication between controllers, actuatorsand/or sensors may be accomplished using a direct wired point-to-pointlink, a networked communication bus link, a wireless link or any othersuitable communication link. Communication includes exchanging datasignals in any suitable form, including, for example, electrical signalsvia a conductive medium, electromagnetic signals via air, opticalsignals via optical waveguides, and the like. The data signals mayinclude discrete, analog or digitized analog signals representing inputsfrom sensors, actuator commands, and communication between controllers.The term “signal” refers to any physically discernible indicator thatconveys information, and may be any suitable waveform (e.g., electrical,optical, magnetic, mechanical or electromagnetic), such as DC, AC,sinusoidal-wave, triangular-wave, square-wave, vibration, and the like,that is capable of traveling through a medium.

FIG. 4 schematically shows a plan view of an electrical circuit for anintegrated test fixture 100 and selected elements of the battery cellinterconnect board 10 that are described with reference to FIG. 1. Theintegrated test fixture 100 includes elements of the test fixture 75described with reference to FIG. 2, and is configured to monitor andevaluate a plurality of junctions 38 formed between corresponding tabs42 and corresponding busbars 30 of the battery cell interconnect board10. The integrated test fixture 100 includes an electric power supply180, a plurality of electric monitoring devices 185, a mechanicalstress-inducing device 190, an associated controller 195, and aplurality of load resistors 182 that may be placed in series withindividual electrical lead traces 44 of the battery cell interconnectboard 10. A single load resistor 182 is indicated in series with asingle junction 38, electrical lead trace 44, and fuse 46 of the batterycell interconnect board 10, and is monitored via leads 186 and 187.

Each of the electrical lead traces 44 has a distinct characteristicresistance that is determined based upon the length, width and thicknessof the electrical lead trace 44, its pattern on the flexible webmaterial 45, the fuse 46, solder and/or other interfaces, the terminalpin 52 and the junction 38 formed between the tab 42 and the busbar 30by the mechanical fastener 36. As such, the characteristic resistancemay differ between individual ones of the electrical lead traces 44 inthe battery cell interconnect board 10. As such each of the loadresistors 182 is preferably selected to limit current draw such that allthe electrical lead traces 44 of the battery cell interconnect board 10have approximately the same current draw when all of the junctions 38have been fabricated in accordance with the specification. As such, thevoltage changes measured across the plurality of junctions 38 of theplurality of the electrical lead traces 44 will be equivalent. Thisconfiguration facilitates employing a single voltage threshold formonitoring all of the electrical lead traces 44 to detect whether thejunctions 38 function in accordance with specification. This facilitatesdetecting a change in the monitored voltage that may occur due to achange in the overall resistance of the electrical lead trace 44,wherein the change in the monitored voltage may be caused by a change inthe resistance across the junction 38. The monitored leads 186 and 187are monitored via a voltmeter 185, which measures the voltage dropacross the junction 38. The voltmeter 185 may be a stand-alone devicethat communicates with the controller 195, or may be electricallyintegrated into the controller 195. Connections to the leads 186 may bemade via a pogo pin, and connections to the leads 187 may be made viathe corresponding terminal pins 152 in the electrical connector 150.Although not shown in detail, the integrated test fixture 100 includesleads 186 and 187 and associated voltmeter 185 for each of the junctions38 of the battery cell interconnect board 10. Overall, the integratedtest fixture 100 is configured to apply mechanical stress to each of thejunctions 38 of the battery cell interconnect board 10 employing themechanical stress-inducing device 190 and monitor the electricalconnections across the monitored leads 186 and 187 via the controller195. The mechanical stress may be applied serially or simultaneously tothe junctions 38. The controller 195 includes circuits and/or controlroutines that monitor signals from the voltmeter 185 to evaluate thebattery cell interconnect board 10. The controller 195 determines thatthe battery cell interconnect board 10 has been fabricated in accordancewith specifications only when the all of the junctions 38 function inaccordance with specification, as indicated by the voltage dropsthereacross.

Embodiments in accordance with the present disclosure may be in the formof an apparatus, a method, or a computer program product. Accordingly,the present disclosure may take the form of an entirely hardwareembodiment, an entirely software embodiment (including firmware,resident software, micro-code, etc.), or an embodiment combiningsoftware and hardware aspects that may all generally be referred toherein as a “module” or “system.” Furthermore, the present disclosuremay take the form of a computer program product embodied in any tangiblemedium of expression having computer-usable program code embodied in themedium.

The detailed description and the drawings or figures are supportive anddescriptive of the present teachings, but the scope of the presentteachings is defined solely by the claims. While some of the best modesand other embodiments for carrying out the present teachings have beendescribed in detail, various alternative designs and embodiments existfor practicing the present teachings defined in the appended claims.

The invention claimed is:
 1. A test fixture for evaluating a junctionbetween an electrical lead trace and a busbar, comprising: an electricpower supply disposed to supply electric power to the electrical leadtrace; an electric monitoring device disposed to monitor electricalpotential across the junction between the electrical lead trace and thebusbar; and a mechanical stress-inducing device disposed to apply amechanical stress proximal to the junction; wherein the electricmonitoring device monitors the electrical potential across the junctionof the electrical lead trace coincident with the mechanicalstress-inducing device applying mechanical stress proximal to thejunction when the electric power supply is supplying electric power tothe electrical lead trace; and wherein the electric monitoring devicedetermines the electrical integrity of the junction based upon themonitored electrical potential across the junction.
 2. The test fixtureof claim 1, wherein the mechanical stress-inducing device disposed toapply a mechanical stress proximal to the junction comprises a nozzle ofa high-pressure source that is aimed towards the junction.
 3. The testfixture of claim 2, wherein the nozzle of the high-pressure source isdisposed to apply a burst of pressurized airflow to the junction.
 4. Thetest fixture of claim 1, wherein the mechanical stress-inducing devicedisposed to apply a mechanical stress proximal to the junction comprisesa hammer device that is disposed to tap on the junction.
 5. The testfixture of claim 4, wherein the hammer device is disposed to apply adirect mechanical tapping onto the junction.
 6. The test fixture ofclaim 1, wherein the mechanical stress-inducing device disposed to applya mechanical stress proximal to the junction comprises an ultrasonicwelding device including a horn that is placed in contact with thejunction.
 7. The test fixture of claim 5, wherein the horn of theultrasonic welding device is disposed to induce vibration at thejunction.
 8. The test fixture of claim 1, wherein the electricmonitoring device is disposed to determine that the junction has beenformed in accordance with its specification when the electricalconductivity across the electrical connector and the busbar is greaterthan a threshold conductivity during the induced mechanical stress. 9.The test fixture of claim 1, wherein the electric monitoring device isdisposed to determine that the junction has not been formed inaccordance with its specification when the electrical conductivityacross the electrical connector and the busbar is less than a thresholdconductivity during the induced mechanical stress.
 10. A method forevaluating a junction that is formed between an electrical lead traceand a busbar, the method comprising: connecting an electrical testfixture disposed to monitor electrical conductivity across theelectrical connector and the busbar; applying an electrical potentialacross the electrical connector and the busbar; inducing a mechanicalstress proximal to the junction and coincidently monitoring theelectrical conductivity across the electrical connector and the busbaremploying the electrical test circuit; and evaluating integrity of thejunction based upon the monitoring of the electrical conductivity acrossthe electrical connector and the busbar when inducing the mechanicalstress.
 11. The method of claim 10, wherein evaluating integrity of thejunction comprises determining that the junction has been formed inaccordance with its specification when the electrical conductivityacross the electrical connector and the busbar is greater than athreshold conductivity when inducing the mechanical stress.
 12. Themethod of claim 10, wherein evaluating integrity of the junctioncomprises determining that the junction has not been formed inaccordance with its specification when the electrical conductivityacross the electrical connector and the busbar is less than a thresholdconductivity when inducing the mechanical stress.
 13. The method ofclaim 10, wherein applying a mechanical stress proximal to the junctionbetween the electrical lead trace and the busbar comprises applying aburst of pressurized airflow to the junction.
 14. The method of claim10, wherein applying a mechanical stress proximal to the junctionbetween the electrical lead trace and the busbar comprises mechanicallytapping onto the junction.
 15. The method of claim 10, wherein applyinga mechanical stress proximal to the junction between the electrical leadtrace and the busbar comprises inducing a mechanical vibration at thejunction.
 16. A test fixture for evaluating a battery cell interconnectboard, wherein the battery cell interconnect board includes a pluralityof junctions between a plurality of electrical lead traces and acorresponding plurality of busbars, comprising: an electric power supplydisposed to supply electric power to the plurality of electrical leadtraces; a controller including an electric monitoring device, whereinthe controller is disposed to monitor electrical potential across eachof the junctions between the electrical lead traces and the busbars; anda mechanical stress-inducing device disposed to apply mechanical stressproximal to each of the junctions; wherein the controller is disposed tocontrol the mechanical stress-inducing device to apply mechanical stressproximal to the junctions when the electric power supply is supplyingelectric power to the electrical lead trace; and wherein the controlleris disposed to monitor, via the electric monitoring device, theelectrical potentials across the junctions of the electrical leadtraces.
 17. The test fixture of claim 16, wherein the controller isdisposed to control the mechanical stress-inducing device tosequentially apply mechanical stress proximal to the junctions.
 18. Thetest fixture of claim 16, wherein the controller is disposed to controlthe mechanical stress-inducing device to simultaneously apply mechanicalstress proximal to the junctions.
 19. The test fixture of claim 16,wherein the mechanical stress-inducing device disposed to apply amechanical stress proximal to the junction comprises a nozzle of ahigh-pressure source that is aimed towards the junction, wherein thenozzle of the high-pressure source is disposed to apply a burst ofpressurized airflow to the junction.
 20. The test fixture of claim 16,wherein the mechanical stress-inducing device disposed to apply amechanical stress proximal to the junction comprises a hammer devicethat is disposed to tap on the junction, wherein the hammer device isdisposed to apply a direct mechanical tapping onto the junction.